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COUPLING TERRESTRIAL AND MARINE

DATASETS FOR COASTAL HAZARD

ASSESSMENT & RISK REDUCTION IN CHANGING

ENVIRONMENTS

COORDINATING CENTRE: ICoD, Malta

PARTNER CENTRES: CERG, France, UNIMORE, Italy

REPORT ON THE RESULTS OBTAINED WITHIN THE COORDINATED

PROJECTS FOR 2015

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ICoD

The focus of the coastal monitoring programme in 2015 concerned the aeolian beach sediment transport study to

compliment the sediment dynamics study initiated in 2014.

Being the main agent of beach erosion, wave erosion tends to overshadow wind erosion. In fact, most studies done

locally leave out completely the element of aerial sediment transport. This study aims at shedding light on the

importance of wind forces in coastal processes. This quantitative study provides the estimated amount of sand being

transported on and off the beach in relation to other factors such as wind speed and direction and other weather

phenomena, as well as dunal vegetation, along established transects, being representative of the whole bay.

The study was conducted between September 2014 and February 2015, and the sand traps were installed in the first

week of September 2014 (Figure 1). Readings for the volume of sand (Table 1) were taken according to specific wind

speed band chosen. Tentative threshold wind velocity for optimum sand transport was determined using both

journals documenting threshold wind velocities in other countries and local meteorological statistics.

The hypothesis being posed in this study is that a large volume of sand at Ramla Bay is being depleted from the

beach and deposited on the backshore by aeolian transport.

Collection no.

Collection Period

Wind Speed (m/s)

Total Weight /

collection

(g)

1

13.09.14 - 14.09.14

1.52

11.87

2

14.09.14 - 17.10.14

1.67

2.3742

3

17.10.14 - 18.10.14

0.83

0

4

18.10.14 - 01.11.14

2.86

5633.50

5

01.11.14 - 15.11.14

2.39

520.11

3

6

15.11.14 - 10.12.14

1.67

128.16

7

10.12.14 - 26.12.14

2.05

125.21

8

26.12.14 - 07.01.15

2.64

3451.62

9

07.01.15 - 13.02.15

2.68

10352.37

Table 1: Analysis of the relationship between weight of sand collected and average wind speed

Figure 1: Sand-trap design and installation pattern at Ramla Bay, Gozo, Malta

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Figure 2: Rate of transport ( m3 m-1 m-1), for every wind direction in black; Resultant rate in blue

With regards to coastal hazard susceptibility, a literature review on ‘mapping’ reveals a dearth of material specifically

related to coastal erosion, the material available being largely limited to landslide susceptibility. Of the relevant

material identified specifically for coastal erosion and sea level rise susceptibility mapping, (Sharples, 2006; Reinen-

Hamill et al., 2006; Abuodha &Woodroffe 2006; Harvey & Woodroffe 2008; Sharples & Carley, 2008 and Sharples et

al., 2013; 2014), the Three pass approach to coastal risk assessment presented by Sharples et al., 2008 and in

subsequent documents was identified by this review as one of the more cost-effective and practical methodology for

multi-scale and multi geographical consideration of coastal erosion susceptibility assessments taking into account

climate and sea-level changes.

Given that:

a susceptibility map is one that provides an indication of where a hazard is more likely to occur by

combining different factors contributing to the hazard;

the term susceptibility may be understood as the capacity of vulnerable elements to withstand actions

of given severity;

it follows that coastal erosion susceptibility mapping should be concerned with the physical sensitivity of the coast to

hazards such changing wave climates and sea level regimes (i.e. the inherent capacity of individual coastal segment

structure to preserve its physical integrity and functionality in the course of physical interaction with coastal erosion

processes or processes that influence coastal erosion (sea-level rise, wave climate, wind and storm climates, tidal

regimes , vertical land movement).

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Based on the Three-pass approach to coastal risk assessment proposed by Sharples & Carley (2008), the following

susceptibility mapping of coastal erosion is proposed:

STEP 1. Using desk and field studies, identify within the shoreline being investigated, the two basic shoreline types

reflecting extreme poles of shoreline susceptibility to coastal erosion, namely, the:

unconsolidated (mud, sand, pebble, cobble beaches);

consolidated (hard) bedrock shoreline stretches;

Geological maps are normally most readily available for use as base maps, but wherever possible, geomorphological

maps are considered more appropriate; work should be carried out to at least 1:250,000 and preferably 1:100,000

scale.

STEP 2. This involves a subdivision of the unconsolidated shorelines into:

• swell-exposed open coast beaches;

• sheltered sandy shores;

a subdivision of the consolidated shorelines into:

• Hard rock shorelines, further subdivided into more stable sloping and and less stable cliffed hard rock

shores;

and

• Soft rock shorelines, further subdivided into more stable sloping and less stable cliffed soft rock shores;

Where according to Sharples & Donaldson, (2014) soft rock is considered to be composed of ‘’semi-lithified

sediments and deeply weathered formerly ‘hard’ bedrock together with cohesive clayey materials which are more

resistant to erosion than soft sediment, but not as resistant as well-lithified rock; these may erode slowly but

significantly over time, and do not rebuild as soft sediment shores may’’.

STEP 3: Rank the above shoreline sub-divisions into broadly-defined categories of greater or lesser susceptibility to

erosion.

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LEAST

SUSCEPTIBLE

SWELL EXPOSED OPEN

COAST BEACHES

SLOPING ROCKY

SHORES

SLOPING SOFT

ROCK CLIFFS

SOFT ROCK CLIFFS

SOFT ROCK

SHORELINES COASTLINE

SHELTERED SANDY

BEACHES

UNCONSOLIDATED

SHORELINES

HARD

ROCK CLIFFS HARD ROCK

SHORELINES

MOST

SUSCEPTIBLE

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UNIMORE

During the second year of the Project the UNIMORE Team has continued the study of active landslides along the

north-west coast of Malta which represent the major geohazard of the area. Extensive rock spreads, which affects

limestone plateaus overlying clayey terrains, evolve into widespread block slides, and favour the occurrence of rock

falls at the plateau cliffs; in addition earth flows/slides affecting the clayey slopes. In particular, the monitored sites of

Il-Prajjet (Figs. 1 and 2) and Il-Qarraba and provided useful inputs for the development of a landslide susceptibility

model which has been applied all over the north-west coast of Malta. The model has also taken into account the

forecasted sea-level rise according to IPCC scenarios. It should be noted that the IPCC Report 2014 proposes the

RCP2.6 scenario as the most probable that forecasts a sea level rise in 2100 of 0.40-0.15 m with a rate of 4.4 mm/yr.

Also thanks to Permanent Scatterer Interferometry (PSI) analyses, a landslide susceptibility map of the north-western

coast of Malta was produced (Fig. 3) through the model Weight of Evidence (WofE) after having outlined the

predisposing factors of landslides in the area: i) curvature, ii) slope angle, iii) Topographic Position Index calculated

for radius of 3, 5 and 10 m, iv) distance of the Permanent Scatterers from scarps, joints, faults and coastline

(Piacentini et al., 2015).

Regarding block slides, the susceptibility map produced through the WofE model was exploited and crossed with an

index representing the ratio between the thickness of Upper Coralline Limetone and the thickness of Blue Clay, which

appeared to be a key parameter, providing the means for a sound assessment of landslide hazard that may be

carried out in further research.

Fig. 1 – Results of GPS monitoring at Il-Prajjet, The arrows represent the direction of the planar deformations

and their length and colour the displacement rate. The newly positioned benchmarks 109 and 110 showed

evidence of activity in an area which was supposed to be stable.

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Fig. 2 – Results of GPS monitoring at Il-Prajjet, The size and colour of the circles represent the rate of the

vertical deformation. The newly positioned benchmarks 109 and 110 do not show evidence of vertical

displacement.

Fig. 3 – Landslide susceptibility map of the north-west coast of Malta (after Piacentini et al., 2015).

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The research explored also the existence of methods or protocols for offshore monitoring of landslides. It resulted

that this type of monitoring in not performed in the study of submarine landslides, mainly of technical reasons (likely

decay of instruments and difficulties in their maintenance). At the present, the only feasible tool for such type of

monitoring consists in repeated bathymetric surveys, such as those that are going to be realised in the Venice lagoon

in order monitor the changing channel morphology.

References

IPCC, 2014. Climate Change 2014 – Synthesis Report. Contribution of Working Group I, II, III to the Fifth

Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge,

United Kingdom and New York, NY, USA, pp. 1-32.

Piacentini D, Devoto S, Mantovani M, Pasuto A, Prampolini M, Soldati M (2015) Landslide susceptibility modeling

assisted by Persistent Scatterers Interferometry (PSI): an example from the northwestern coast of Malta. Natural

Hazards, 78, 681-697.

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CERG*

* With University of Caen Normandie (LETG UNICAEN, France), University of Brest (IUEM-DO, Brest, France),

University of Lausanne (CRET, UniL, Lausanne, Switzerland).

Description: Analysis of the monitoring programme results.

During the second year of the Project the CERG Team has continued to study and monitor the hard rock cliff, in

Upper Normandy, subjected to landslides (cliff falls, debris fall and boulder and rock falls) and storm surges, in each

part of Dieppe arbour from Cap d’Ailly (Varengeville) to Puys and the cliffs of ‘Les vaches noires’ subjected to

landslides and muddy flows in Lower Normandy from Villers-sur-mer to Houlgate.

In September 2015, a simultaneous MLS (Mobile Laser Scanning), TLS (Terrestrial Laser Scanning), Unmanned

Aerial Vehicle (UAV) photogrammetric survey and Terrestrial photogrammetric survey campaigns was carried out in

order to acquire new data on subaerial landforms. The main objectives were: to compare the data acquired by these

different techniques in the same zones, in order to define their respective resolution and repeatability; to verify if the

MLS and UAV methods could become a reliable and rapid techniques for regularly monitor the cliff dynamic; to define

the protocols which can be used in other coastal environments for hazard assessment.

Time-wise, UAV perform much faster to acquire stereo-photographs of a larger surface. Shooting strategy, adding

oblique photographs in addition to strictly normal shots, is also important to control the distortion of 3D models (Fig.

1). Finally, the quality of the 3D dense reconstruction is very dependent upon software processing and its settings.

.

Fig. 1 – DTM obtained by UAV photogrammetric survey in Ailly cliff (Upper Normandy). The flags

corresponding to the controlled benchmarks (from LDO).

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In the other test site of Les Vaches noires, the comparison of TLS results from September 2014 to June 2015

confirmed that the TLS techniques is a reliable and rapid techniques for regularly monitor the cliff dynamic. On the

example of frontal view (Fig. 2), we obtained a very good general aspect of the landforms. In high density areas, the

resolution is better than 5 cm. The morphological parameters can be well observed, even on intertidal areas.

Surfaces processes, such as collapse talus on the upper part and mudflows on the lower part (accumulation) can be

observed and quantified (Maquaire et al., 2015).

Fig. 2 - Comparison of DTMs (5cm) of September 2014 and June 2015 obtained by Terrestial Laser Scanning

(TLS) at the Vaches noires cliff (in Maquaire et al., 2015)

Description: Exploration of methods for the selection of the most appropriate techniques to perform offshore

landslide monitoring

Like our Unimore colleagues, we have explored the different existing methods or protocols for offshore monitoring of

landslides in particular and cliff evolution in general. We arrived at the same conclusion. If the feasible tool for such

type of monitoring consists in repeated bathymetric surveys, sometimes, this type of prospection with multibeam

sounder is not always possible due to low deep of water, presence of dangerous blocks, … Also, the new

bathymetric Aerial laser Scanning devices used green rays could be providing continuous terrestrial and submarine

datasets. Due to technical problem the test planned in 2015 has been postponed in 2016.

References

Maquaire O., Costa S., Thomas A., Medjkane M., Letortu P., Davidson R., Roulland Th., Fressard M. (2015). Suivi de

l’évolution des falaises des vaches noires par laser terrestre (TLS) et photogrammétrie (Calvados, Basse-Normandie,

France). Journées ‘Aléa gravitaire’ JAG 2015, Caen, 3-4 septembre, 20 p.